Emulsions: Principles and preparation - Exercises

9 important questions on Emulsions: Principles and preparation - Exercises

How can a W/O/W double emulsion be made?

For the primary W/O emulsion, we should probably use a high pressure homogeniser or a rotor-stator device at high speed. These techniques create relatively high shear (especially the high pressure homogeniser) so the required droplet size can be reacher.

Then, for making the final W/O/W emulsion, we need a milder technique, otherwise the emulsion will be damaged. A rotor-stator homogeniser used at low speed could work, or (better) premix membrane emulsification.

The following figure A shows the particle size distribution in 3 protein-stabilised emulsions. Figure C shows the particle size distribution of the same emulsions after dilution (and vortex) in a concentrated solution of sodium dodecyl sulphate (SDS). How would you explain the differences between Figures A and C?

SDS will absorb at the interface of the particles, giving electrostatic repulsion and smaller droplets.

In practice, the volume fractions and the composition in a phase diagram will not always be obtained. Why?

The phase diagram gives information about the system when it is in thermodynamic equilibrium. However, in practice there might be kinetic limitations as a result of which the system does not reach the thermodynamic equilibrium.

How will the binodal shift upon heating?

  • Heating leads to increased solubility --> curve will go up and to the right
  • However, in the case of proteins, heating leads to denaturation and due to protein aggregation, protein solubility might decrease --> curve will go down and to the left

Why does the binodal shifts when the pH is decreased from pH 7 to pH 3 for whey proteins?

At pH 7 the whey protein isolate is present as dimers. When changing to pH 3, the dimers change to monomers and thus the average molecular weight decreases. When the average molecular weight is lower, phase separation will take place at higher concentrations and thus the binodal will shift up/to the right when changing from pH 7 to pH 3.

How will the binodal curve shift if the dextran is replaced by a dextran with a higher molecular weight?

The binodal shifts down. When dextran has a higher molecular weight, it is less soluble and phase separation takes place at lower concentrations.

Can you explain why the glass transition temperature was decreased as a result of the thermo mechanical treatment inside the extruder?

During extrusion, material is thermo mechanically processed. Shear as well as thermal treatment can lead to a certain extend of depolymerisation of the starch chains. A reduction in starch chain length leads to a reduction of the glass transition temperature.

Why does a higher screw speed lead to a higher solubility?

This will lead to more friction followed by molecular breakdown, and increased solubility.

Oil is dispersed in the continuous phase. Does oil contribute to the anisotropy of the final product?

The interfacial tension between oil and water is higher than the interfacial tension between the two aqueous phases. The Gibbs free energy is higher at higher interfacial tensions: more energy is needed to enlarge the interfacial area and thus droplets remain spherical and are not elongated into fibres. Therefore, it will not contribute to the anisotropy.

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